Ninja,Makefile: Drop now-unused placeholders from link rule variables

Previously we used separate placeholders for builtin linker flags:

* CMAKE_<LANG>_LINK_FLAGS for EXECUTABLEs
* CMAKE_SHARED_LIBRARY_CREATE_<LANG>_FLAGS for SHARED libraries
* CMAKE_SHARED_MODULE_CREATE_<LANG>_FLAGS for MODULE libraries

These are now always replaced by the empty string, so drop them
from our rule variables.